This invention relates to the production of chlorophosphazene mixtures, particularly to such mixtures having a high proportion of cyclic trimeric and tetrameric chlorophosphazene. Such products are useful in the preparation of inorganic rubber, as intermediates for agricultural chemicals, as intermediates for fire retardants and as coatings for ceramics and metals.
Because of the various end uses for chlorophosphazenes, extensive research effort has recently been involved in processes for their production. Efforts have been placed on two routes to obtain high cyclic content chlorophosphazenes -- i.e., through control of the reaction itself and through post-treatment of the reaction product.
Post treatment of the reaction product has the disadvantage of requiring additional process equipment and higher processing costs, as well as inefficient utilization of reactants and solvents. Control of the reaction itself appears to offer the best route to efficient and economical production of the desired cyclic chlorophosphazene trimer and tetramer mixtures.
The prior art teaches several general approaches to achieve the desired end product through control of the reaction. For example, high dilution of the reactants appears to favor increased cyclic content, Allcock, Phosphorus-Nitrogen Compounds, Academic Press, New York (1972), p 122. Also, the use of an excess of finely divided ammonium chloride particles favors increased cyclic content; see U.S. Pat. No. 3,367,750. Further, the slow and even addition of one reactant to the other appears to favor higher cyclic contents, although the prior art differs somewhat on this point. One line of patents indicates that slow and even addition of PCl.sub.5 to NH.sub.4 Cl favors increased cyclic content; see U.S. Pat. Nos. 3,667,922 and 3,367,750. Another group prefers the addition of NH.sub.3 to a solution of PCl.sub.5 to increase cyclics; see U.S. Pat. Nos. 3,656,916 and 3,658,487.
Such process conditions provide advantages over classical techniques in which cyclic contents of chlorophosphazene mixtures ranged from 40 to about 65 weight percent cyclics, increasing the cyclic content to from 65 to about 90 weight percent. However, the use of dilute systems has the disadvantage of requiring large volumes of solvent and increased investment required to store, handle and recover the solvent for reuse. Also, the preparation of all of the finely divided ammonium chloride for subsequent reaction makes a thick difficultly stirred slurry. Further, slow additions of the reactants requires increased cycle times. Finally, the cyclic contents produced in such prior art reactions could still be improved.
More recently the yields of cyclic content have been increased by the use of metallic halide catalysts. In U.S. Pat. No. 3,860,693 catalysts such as cobaltous chloride, aluminum chloride, manganous chloride, cupric chloride, stannic chloride magnesium chloride, zinc chloride and titanium chloride are mentioned as effecting increased cyclic content.
Also, U.S. Pat. No. 3,780,162 teaches a process for reacting PCl.sub.5 and NH.sub.4 Cl wherein an amount of the NH.sub.4 Cl sufficient to initiate the reaction is prepared by adding ammonia and hydrogen chloride under pressure and then subsequently adding PCl.sub.5 and ammonia to the reactor at specified rates to continue the reaction. The rate of ammonia feed is initially high compared to the phosphorus pentachloride so that large concentrations of PCl.sub.5 are not built up. Later the ammonia feed is slowed. Although this process has advantages over other processes, it has the disadvantage that precise control of the feed rates must be maintained to achieve the desired results. Moreover, even when such precise control is achieved a substantial amount of linear species is produced. Thus, there is a need for a process which overcomes the disadvantages of the prior art.